1. Technical Field
A hot plate oven is used in a thermal process for forming an ultrafine photoresist pattern. More particularly, a hot plate oven is used to flow photoresist under high pressure in the sealed oven during a resist flow process.
2. Description of the Related Art
Resist flow is a processing technology for forming a fine pattern which exceeds the resolution of the exposing device.
The resist flow process has recently made remarkable developments and so that it is now used in mass production processes. The technology generally involves an exposure process and a development process. The process forms a first photoresist pattern having a resolution equal to that of the exposing device. The process also includes heating the first photoresist pattern to a temperature higher than the glass transition temperature of the photoresist. As a result, the first photoresist pattern is flowed by the supplied heat energy, reducing size of the first photoresist pattern and a second photoresist pattern smaller than the first photoresist pattern is finally obtained.
A method of forming a photoresist pattern using a conventional resist flow process is illustrated in FIGS. 1a-1d. As shown in FIG. 1a, a photoresist film 12 is first formed on a semiconductor substrate 22. The photoresist film 12 is exposed using a mask 10 with uniform patterns a, b and c that are subsequently developed. In the case of a positive photoresist, the exposed regions 14 of the photoresist film 12 are removed while the unexposed regions 16 of the photoresist film 12 remain. As a result, a first photoresist pattern 18 is obtained, having the same size of patterns (a=b=c), as shown in FIG. 1b. 
Thereafter, as shown in FIG. 1c, the semiconductor substrate 22, with the first photoresist pattern 18 formed thereon, is put into the hot plate oven 100. Photoresist is then flowed by heating the first photoresist pattern over the glass transition temperature. And, the supplied heat energy generates a force F1 which reduces the size of pattern. Therefore, the size of the first photoresist pattern 18 is increased and the size of the gaps is then reduced by the force F1. A second photoresist pattern 20 is finally obtained (see FIG. 1dwith smaller gaps a′, b′, c′), which requires an integrated process.
However, as shown in FIG. 1c, the above-described conventional hot plate oven 100 comprises a hot plate 102, a gas inlet 104 and a gas vent 106. During the resist flow process using this hot plate oven 100, the flow X of gas is then generated since gas is injected via gas inlets 104 and exhausted via a gas vent 106 thereby increasing non-uniformity of the bake temperature. As a result, the instability of the process increases because the sizes of the second photoresist patterns 20 are not uniform.
As shown in FIG. 1d, the size of the gaps a′, b′, c′ of the second photoresist pattern is smaller than the size of the gaps a, b, c of the first photoresist pattern 18 and the pages a′, b′, c′ are not uniform (a′≠b′≠c′).
FIG. 2 is a graph illustrating the size change of photoresist pattern as a function of the bake temperature during a resist flow process using a conventional hot plate oven. As shown in FIG. 2, as the temperature goes up, the size of photoresist pattern is drastically reduced. Accordingly, it is difficult to keep the size of the patterns over 50 nm when the baking temperature exceeds 130° C.